Gun barrel liner, and additive method of making

ABSTRACT

A method making a gun barrel liner includes depositing material on a mandrel that has ribs or ridges corresponding to the rifling grooves in the liner. The material may be deposited using plasma spraying, with sprayed splats of material re-melted after deposition for at least part of the deposition process to reduce or eliminate porosity in the deposited material. The mandrel may be made of metal, such as a high thermal conductivity metal such as copper. The mandrel may have a channel therein or therethrough. The channel may facilitate flow of liquid though the mandrel, one example of such a liquid being a coolant (such as water), used to remove heat produced during the material deposition. Another liquid passed through the channel may be an etchant (or other fluid) that is used to dissolve or otherwise remove the mandrel after the material of the gun barrel liner has been deposited.

FIELD OF THE INVENTION

The invention is in the field of gun barrels, liners for gun barrels, and methods of making gun barrels and gun barrel liners.

DESCRIPTION OF THE RELATED ART

Gun barrels are used in a variety of situations, for firing various types of bullets, shells, rocket assisted/propelled projectiles and other munitions. These gun barrels are subject to harsh use, with environments including high pressure, high temperature, and corrosive gases, from repeated firings of the gun. Wear and life are therefore a concern with regard to gun barrels.

Gun barrels are often rifled, with spiral groves on the inside of the gun barrel used to spin a munition projectile as it is fired, improving accuracy and/or range. The problem of how to produce rifling has been addressed in many different ways, including difficult machining processes for modifying the inner surface of the gun barrel, and using methods such as electrochemical deposition to selectively deposit material onto the inner surface of the gun barrel. These methods have proved in general not fully satisfactory, such as by being costly and environmentally undesirable or difficult to perform, or by producing gun barrels subject to wear and/or underperformance.

In addition there is a more general problem of erosion of gun barrels (both rifled and smooth-bore) through repeated firings of a weapon. There is a need for improvements in the prevention of erosion of gun barrels, such as by improving the erosion resistance of the inner surface of the barrel.

In view of the above, room for improvement in this field of endeavor exists.

SUMMARY OF THE INVENTION

A method of making a gun barrel includes depositing on a mandrel material for a liner of the gun barrel.

A method of making a gun barrel liner includes depositing material on a mandrel having spiral ribs or ridges on its outer surface, the ribs or ridges corresponding to rifling for the gun barrel liner.

According to an embodiment of any paragraph(s) of this summary, the depositing of material may be by plasma deposition.

According to an embodiment of any paragraph(s) of this summary, the plasma deposition may involve re-melting of deposited splats or droplets of material, to thereby decrease porosity of at least part of the gun barrel liner.

According to an embodiment of any paragraph(s) of this summary, a protective layer may be formed on the mandrel prior to the plasma deposition, to keep the mandrel surface from melting/softening from heat from the plasma deposition or from re-melting of the initial deposition layers.

According to an embodiment of any paragraph(s) of this summary, the protective layer may be a high-melting-temperature material deposited onto the outer surface of the mandrel.

According to an embodiment of any paragraph(s) of this summary, the protective layer may be deposited by chemical vapor deposition.

According to an embodiment of any paragraph(s) of this summary, the protective layer may be deposited by electroplating.

According to an embodiment of any paragraph(s) of this summary, the mandrel may be actively cooled during the plasma deposition.

According to an embodiment of any paragraph(s) of this summary, the cooling may be forced convection/conduction cooling that includes pumping a liquid coolant, such as water, through a channel, modified channel or multiple channels in the mandrel.

According to an embodiment of any paragraph(s) of this summary, the mandrel may be etched away or dissolved after formation of the gun barrel liner. This may include passing an etchant through the channel.

According to an aspect of the invention, a method of making a gun barrel liner with rifling includes the steps of: depositing material onto a ridged outer surface of the mandrel to form the gun barrel liner, wherein the ridged outer surface corresponds to the rifling; and while depositing the material, cooling the mandrel by passing coolant through a channel in the mandrel.

According to an embodiment of any paragraph(s) of this summary, the channel is along a longitudinal axis of the mandrel; and the cooling includes passing the coolant from one longitudinal end of the mandrel to an opposite longitudinal end of the mandrel.

According to an embodiment of any paragraph(s) of this summary, the coolant is water. Alternatively the coolant may be ethylene glycol, propylene glycol, brine, oil, or polymer/water solutions and combinations of these.

According to an embodiment of any paragraph(s) of this summary, the method further includes, following the depositing, chemically removing the mandrel.

According to an embodiment of any paragraph(s) of this summary, the chemically removing includes passing an etchant through the channel.

According to an embodiment of any paragraph(s) of this summary, the depositing includes plasma deposition.

According to an embodiment of any paragraph(s) of this summary, the depositing includes laser or e-beam sintering/re-melting.

According to an embodiment of any paragraph(s) of this summary, the depositing includes re-melting at least some of the material after the material is deposited, thereby producing re-melted material with reduced porosity of the material.

According to an embodiment of any paragraph(s) of this summary, the re-melting includes re-melting inner material of the liner having a thickness at least that of an expected wear thickness of the liner.

According to an embodiment of any paragraph(s) of this summary, the depositing includes plasma deposition of additional material onto the re-melted material, where the additional material has greater porosity than the re-melted material.

According to an embodiment of any paragraph(s) of this summary, the method further includes, prior to the plasma deposition, depositing a protective material onto the mandrel, with the protective material having a higher melting temperature than material of the mandrel.

According to an embodiment of any paragraph(s) of this summary, the depositing the protective material includes chemical vapor deposition of the protective material.

According to an embodiment of any paragraph(s) of this summary, the depositing material includes depositing a material selected from the group consisting of chromium and alloys of chromium including nickel and/or cobalt (as well as other possible metals, such as molybdenum).

According to an embodiment of any paragraph(s) of this summary, an inner layer thicker than the projected worst-case eroded layer is deposited on a mandrel of more expensive but more capable material such as tantalum, under a thicker layer of less expensive material such as chromium. This is advantageous for extreme situations involving extremely high temperature/pressure propellants.

According to an embodiment of any paragraph(s) of this summary, the method of making the gun barrel liner is part of a method of making a gun barrel, the method of making the gun barrel further including: mechanically coupling the gun barrel liner to a cylindrical jacket that is outside of and surrounds the liner.

According to another aspect of the invention, a gun barrel liner includes: an inner layer of material with relatively low porosity; and an outer layer of material, surrounding the inner layer, wherein the outer layer has relatively high porosity. The inner layer of material has rifling grooves for rifled barrels.

According to an embodiment of any paragraph(s) of this summary, the inner layer and the outer layer are made of the same material.

According to an embodiment of any paragraph(s) of this summary, the layers are formed by plasma deposition.

According to an embodiment of any paragraph(s) of this summary, the inner layer is formed by re-melting the material of the inner layer.

According to yet another aspect of the invention, a method of making a gun barrel liner with rifling includes: depositing material onto a ridged outer surface of the mandrel to form the gun barrel liner, wherein the ridged outer surface corresponds to the rifling; wherein the depositing includes depositing the materials with different porosity, with inner material of relatively low porosity (less porous) and outer material of relatively high porosity (more porous).

According to an embodiment of any paragraph(s) of this summary, the depositing includes plasma deposition; and the depositing includes re-melting the inner material.

According to an embodiment of any paragraph(s) of this summary, the inner material and the outer material have the same material composition.

To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

The annexed drawings, which are not necessarily to scale, show various aspects of the invention.

FIG. 1 is a schematic view of a system for producing or making a rifled gun barrel liner by an additive manufacturing process, according to an aspect of the present invention.

FIG. 2 is a high-level flow chart of a method for making a rifled gun barrel liner, using the system of FIG. 1.

FIG. 3 is a cross-sectional view of a gun barrel liner according to an embodiment of the invention.

FIG. 4 is a cross-sectional view of a gun barrel that includes a liner according to an embodiment of the invention.

DETAILED DESCRIPTION

A method making a gun barrel liner includes depositing material on a mandrel that has ribs or ridges corresponding to the inverse of the rifling grooves in the liner. The material may be deposited using plasma spraying, with sprayed splats of material re-melted after deposition for at least part of the deposition process, such as for the innermost parts of the liner, to reduce or eliminate porosity in the deposited material. The mandrel may be made of metal, such as a high thermal conductivity metal such as copper, brass, bronze, aluminum, etc., which also has the advantage of being easy to machine, so as to facilitate machining of the ribs or ridges on the outer surface of the mandrel. The mandrel may have a channel therein or therethrough, for example having a channel or bore along a longitudinal axis of the mandrel extending between opposite longitudinal ends of the mandrel. The mandrel may facilitate flow of liquid through the mandrel, one example of such a liquid being a coolant (such as water, glycol, ethylene glycol, propylene glycol, brine, oil, polymer/water solutions. or a suitable liquid metal), used to remove heat produced during the material deposition. Another liquid passed through the channel may be an etchant (or other fluid) that is used to dissolve or otherwise remove (at much increased rate) the mandrel after the material of the gun barrel liner has been deposited, with the etchant being chosen to remove the liner/barrel material at a much lower rate than the mandrel. Following the formation of the gun barrel liner, the liner may be further treated as necessary (for example by machining or polishing the outside surface of the liner to smooth the outside surface), and then mechanically or otherwise joined to an outer jacket that surrounds the liner. The liner may be made of any of a variety of suitable materials, such as chromium or alloys of chromium including predominant alloying agents such as nickel and/or cobalt and important but small ingredients such as molybdenum, titanium, aluminum or yttrium.

FIG. 1 shows a system 10 for producing or making a rifled gun barrel liner 12 by an additive manufacturing process, depositing material on a mandrel 14. The mandrel 14 has ribs or ridges 16 on its outer surface 18. The ribs or ridges 16 extend around the outer surface 18 in a spiral configuration, with the configuration of the ribs 16 corresponding to the desired rifling grooves in the liner 12. The mandrel 14 may be made of copper or another suitable material, with the outer surface 18 machined to produce the ribs 16. Suitable material for the mandrel 14 would be a material that has a high thermal conductivity, and allows for easy formation of the ribs 16, such as by machining. Besides copper, suitable materials for the mandrel 14 may include brass, nickel-copper alloys sold under the trade name MONEL, other alloys of copper and nickel such as CuproNickel, and aluminum. As an alternative, the mandrel may be made without ribs, to produce a gun barrel liner without rifling.

The mandrel 14 has a channel 30 therethrough, allowing flow of a fluid through the mandrel 14. The channel 30 may be along a longitudinal axis 31 of the mandrel 14, from one longitudinal end of the mandrel 14 to an opposite longitudinal end. Channels at other locations within the mandrel 14 are possible alternatives. One possible purpose of the channel 30 is to allow flow of a coolant fluid, for example water, during the process of deposition of the material of the liner 12. As explained in greater detail below, the liner material may be deposited using a hot deposition process, such as deposition by a plasma gun 32. In addition heat may be added to the deposited material in order to re-melt the deposited material, thereby reducing the porosity of the deposited material. It would be desirable to remove at least some of this added heat by use of a coolant following through the mandrel 14, for instance to avoid a buildup of heat that could melt or otherwise degrade the mandrel 14, or to increase the rate in which the material can be deposited without raising the temperature of the mandrel to an undesired level. The coolant fluid may be circulated in a closed loop, with a coolant pump 34 used to move the fluid through the loop. A heat exchanger 36 in the loop may be used to remove heat from the coolant.

In the illustrated embodiment the mandrel 14 is depicted as cylindrical, but it will be appreciated that more generally there may be a variation in the diameter of the mandrel 14, in order to produce a liner 12 with a non-uniform central bore diameter. The main part of the mandrel 14 may have a very close to constant diameter, with perhaps a very slight reduction as the position goes from the location of the breach to that of the muzzle. The chamber end may have a large change in diameter to accommodate the case of the ammunition in addition to just the projectile to be fired. In addition the liner 12 may have a non-uniform thickness, for example with the liner 12 being thicker at one end, where the chamber of the gun barrel is located. The channel 30 may have a wider thickness at the chamber end, and the coolant may enter the channel 30 at the chamber end, flowing toward the opposite end of the mandrel 14.

The channel 30 may also be used (or may alternatively be used) as part of the process for removing the mandrel 14, after formation of the gun barrel liner 12. A suitable etchant or other material for dissolving or removing of the material of the mandrel 14 may be passed through the channel 30. This process may be an open or closed-loop process. Alternatively the liner 12 and the mandrel 14 may be immersed in a bath of etching material that preferentially removes the material of the mandrel 14, such as with a directional flow of etchant within the bath. Suitable etchants may include materials such as ferric chloride, hydrochloric acid, and sulfuric acid, to give a few examples. The use of the channel 30 allows an etchant to work faster and better, improving the economic efficiency of the process.

FIG. 2 shows a high-level flow chart of a method 100 for making the gun barrel liner 12 (FIG. 1), using the system 10 (FIG. 1) for at least some of the steps. In step 102 the mandrel 14 (FIG. 1) is provided, for example by using machining to produce the ribs 16 (FIG. 1) and the channel 30 (FIG. 1).

In step 104 material for the liner 12 (FIG. 1) is deposited on the mandrel 14 (FIG. 1). As noted above, the deposition may be a plasma deposition, with a plasma gun or spray torch 32 (FIG. 1) used to heat a powdered material and deposit the powdered material. The material may be any of a variety of materials that would be suitable for lining a gun barrel, for containing the corrosive pressurized high-temperature gases produced from firing of a munition, while preventing excessive wear. During gun firing such gases may be at a pressure of 6.9×10⁸ Pascals (100,000 psi), and at 3,000° C., to give example values, with the liner 12 having to withstand multiple pressure pulses at such a level (for example 100 to 50,000 pulses, depending on the type of weapon). It is also desirable that the material for the liner 12 be inexpensive and easily deposited. Further, the material for the liner 12 should resist removal as the mandrel 14 is removed, for example by not being dissolved by the etchant that is used to remove the mandrel 14.

The material deposition may be accomplished using a suitable plasma deposition method. A suitable plasma-jet spray torch or plasma gun 32 (FIG. 1) may be used to spray a powder that is melted or evaporated, using a plasma jet electrically set up in a reduced pressure chamber. There may be relative movement between the plasma gun 32 and the liner 12 (FIG. 1) and the mandrel 14 (FIG. 1) to provide a planned coating of the material for the liner 12, to build up the liner 12 on the mandrel 14. The deposition may be computer controlled, such as to minimize local heating and/or to control the shape and/or thickness of the deposited shape. For instance a liner with an exterior taper could make insertion into a surrounding gun barrel jacket easier and more exact, for example to facilitate an interference fit between the liner and a surrounding jacket. The interference fit could be exactly provided by relative temperature instead of exacting cylindrical exterior dimensions of the liner and the cylindrical interior dimensions of the barrel structure to be lined.

The plasma deposition is accomplished by depositing the material as liquid droplets, known as splats. Generally in plasma deposition the splats solidify as soon as they are deposited, and later splats do not always completely and exactly fill in the area at the edge/foot of prior splats, which leads to a certain amount of porosity in the material. According to one embodiment of the invention the plasma deposition includes providing sufficient heat during the deposition process to re-melt the deposited splats after their deposition, during at least part of the deposition process, without re-melting a significant layer underneath the deposited splats. The re-melting may be effected by a second plasma gun, or by slowing down movement of the plasma-jet spray deposition torch (relative to the liner 12 and the mandrel 14). As another alternative the re-melting may be accomplished by a second pass of the plasma gun used to deposit the initial material, with no spraying occurring in the second pass. The re-melting reduces the porosity of the resulting material, increasing the resistance of the material to the ingress of gases, such as corrosive gases produced in the firing of a munition, such as a bullet or shell. It is advantageous to reduce the porosity especially in the inner portion of the liner 12 (FIG. 1), the part of the liner 12 that surrounds the axis of the gun barrel where the munition is fired. All of the material of the liner 12 may be re-melted to reduce porosity, or alternatively the re-melting may be limited to only the inner part of the liner 12. This latter situation is illustrated in FIG. 3, an embodiment of the liner 12 in which an inner layer 42 is less porous than an outer layer 44.

The liner 12 may be made of any of a variety of suitable materials, such as chromium or alloys of chromium that include nickel and/or cobalt. Such materials may include nichrome alloys, which are alloys of nickel, chromium, and perhaps other materials (such as iron); and alloys that are sold under the trademark STELLITE, which are alloys of cobalt and chromium that may also contain other materials, such as tungsten and/or molybdenum (among many other additional materials). Tantalum is another possible suitable material for the liner 12.

The same material composition may be deposited for all parts of the liner 12 (FIG. 1). Alternatively there may be variation of the liner material composition or properties in the radial and/or axial direction. For example, the inner-most part of the liner 12 may be made of a material that is corrosion resistant. As another example, the liner may have an inner portion made of a more expensive material than an outer portion, with the inner portion having a thickness of at least a maximum erosion depth for the gun in question. For example, an inner portion of the liner may be made of chromium (or alternately tantalum), up to a thickness of the maximum erosion depth, with the outer part of the liner being made of steel (or alternately chromium). The use of a material such as steel in the outer part of the liner may provide strength and stiffness to the liner that facilitates mechanical combination of the liner and a jacket that surrounds the liner.

During the deposition of the liner material the mandrel 14 (FIG. 1) is cooled, in step 106. As discussed above the pump 34 (FIG. 1) may be used to pump a cooling fluid such as water through the channel 30 (FIG. 1), cooling the mandrel 14 (FIG. 1).

After the deposition the mandrel 14 (FIG. 1) is removed in step 108. The removal may involve etching away all or part of the mandrel 14, such as by pumping a suitable etchant through the channel 30 (FIG. 1), or by placing the mandrel 14 (and the liner 12 (FIG. 1)) in an etchant bath. The use of an etchant to dissolve parts of the mandrel 14 may be supplemented by other methods, to remove other parts of the mandrel 14. As one example of an alternative, it may be possible to use abrasive particles in an air stream may be used to remove some or all of the mandrel.

In step 110 the liner 12 (FIG. 1) may be subjected to further treatments, machining, or processing as necessary in order to prepare it for coupling to the rest of the gun barrel. For example there may be machining, grinding, lapping, or polishing the outside surface of the liner 12 to smooth the outside surface. Also, the inside of some barrels may be lapped to smooth the innermost surface of the liner/barrel.

Finally, in step 112 the liner 12 is mechanically coupled to a jacket 50, as shown in FIG. 4, to make a gun barrel 52. The jacket 50 may be made of a different material than that of the liner 12, for example a less expensive material. The jacket may be made of steel for example, and may be made using more conventional fabrication techniques. Suitable steels for the jacket 50 include carbon steel, 4150 steel, 4340 steel, Elgin steel, and stainless steels such as 416 or 304 stainless steel. The liner 12 (FIG. 1) may be inserted into the jacket 50 and mechanically coupled to the jacket 50 by any of a variety of suitable techniques, such as being brazed or bonded in place or an interference fitting. Placing a relatively cold liner 12 into a relatively hot jacket 50 may be sufficient to secure the two parts together (with the liner 12 in compression) as the parts equalize in temperature.

There are several options to insert a liner for service in a barrel jacket. One method is using an adhesive such as epoxy adhesive, and forced insertion with minimal interference between the liner and the jacket.

Another method of insertion/combination/fabrication is to heat the outer object and cool the inner object before insertion. This expands the aperture and contracts the inner object and lessens or eliminates the interference. This allows the final system to place the inner liner under significant permanent compressive forces/stresses and subjects the outer barrel to significant tensile forces/stresses. This is an advantage in that the lower cost material of the outer barrel can prevent momentary tensile forces/stresses in the liner. A thick inexpensive ductile steel jacket (such as 304 stainless) holding a less ductile liner in compression may allow high pressures in the propellant combustion launching of the projectile, leading to desirable higher velocities without sacrificing barrel life. The interference between the liner and the jacket may be set by a difference in the outer diameter of the cylindrical liner and an inner cylindrical diameter of the jacket. Alternatively or in addition, the liner and the jacket may be conical, with well-matched taper angles (such as within 0.1 degree), with the tapering being to allow the interference fit between the relatively hot jacket and the relatively cold liner to be caused by temperature difference relaxation alone.

Further, at the end of the liner fabrication cycle (either just at the end of liner material deposition, or after precision surface dimension machining) the plasma deposition head already in use for the liner creation may be used to deposit a layer of braze material, for example, as 0.005 mm (0.0002 inch) thick layer of Ag—Cu—Sn braze material. After insertion, the system can be brought to the melt temperature of the braze material for a few minutes, to create a continuous bond between the layers instead of a friction hold (and also to increase the thermal conductivity of the interface).

As an alternative to the plasma deposition in step 104, the material of the liner 12 (FIG. 1) may be deposited using other deposition techniques, such as electroplating, chemical vapor deposition (CVD) or physical vapor deposition (PVD). When CVD is used, porosity may not be much of a concern, and there may be no need for any cooling as part of the process, such as the forced convention cooling of the mandrel 14 (FIG. 1) in step 106.

A non-plasma deposition process such as CVD or electroplating may be used in conjunction with plasma deposition, in order to deposit a thin layer of high-melting-temperature material on the mandrel 14 (FIG. 1), prior to the plasma deposition. The high-melting-temperature material may be chromium, tantalum, or nickel, to give a few examples. The thin layer of material may serve as a protective layer to protect the material of the mandrel 14 from melting, and therefore must have a higher melting temperature than that of the material of the mandrel 14.

The method described herein may be used to make gun barrel liners over a large range of sizes. By way of non-limiting example, the liners made using the method may range from those for 5.56 mm projectile barrels, to those for 155 mm projectile barrels. Although the method is described above for use in making rifled liners, it alternatively may be used to make smooth-bore liners, such as (for example) for tank guns and artillery.

Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application. 

1. A method of making a gun barrel liner with rifling, the method comprising: depositing material onto a ridged outer surface of the mandrel to form the gun barrel liner, wherein the ridged outer surface corresponds to the rifling; and while depositing the material, cooling the mandrel by passing coolant through a channel in the mandrel.
 2. The method of claim 1, wherein the channel is along a longitudinal axis of the mandrel; and wherein the cooling includes passing the coolant from one longitudinal end of the mandrel to an opposite longitudinal end of the mandrel.
 3. The method of claim 1, wherein the coolant is water.
 4. The method of claim 1, further comprising, following the depositing, chemically removing the mandrel.
 5. The method of claim 4, wherein the chemically removing includes passing an etchant through the channel to remove the mandrel.
 6. The method of claim 1, wherein the depositing includes plasma deposition.
 7. The method of claim 6, wherein the depositing includes re-melting at least some of the material after the material is deposited, thereby producing re-melted material with reduced porosity of the material.
 8. The method of claim 7, wherein the re-melting includes re-melting inner material of the liner having a thickness at least that of an expected wear thickness of the liner.
 9. The method of claim 8, wherein the depositing includes plasma deposition of additional material onto the re-melted material, where the additional material has greater porosity than the re-melted material.
 10. The method of claim 6, further comprising, prior to the plasma deposition, depositing a protective material onto the mandrel, with the protective material having a higher melting temperature than material of the mandrel.
 11. The method of claim 10, wherein the depositing the protective material includes deposition of the protective material using one or more of electroplating, chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma, or sputtering.
 12. The method of claim 1, wherein the depositing material includes depositing a material selected from the group consisting of chromium and alloys of chromium including nickel and/or cobalt.
 13. The method of claim 1, wherein the method is part of a method of making a gun barrel, the method of making the gun barrel further comprising: mechanically coupling the gun barrel liner to a cylindrical jacket that is outside of and surrounds the liner.
 14. A gun barrel liner comprising: an inner layer of material with relatively low porosity; and an outer layer of material, surrounding the inner layer, wherein the outer layer has relatively high porosity; wherein the inner layer of material has rifling grooves.
 15. The gun barrel liner of claim 14, wherein the inner layer and the outer layer are made of the same material.
 16. The gun barrel liner of claim 14, wherein the layers are formed by plasma deposition.
 17. The gun barrel liner of claim 16, wherein the inner layer is formed by re-melting the material of the inner layer.
 18. A method of making a gun barrel liner with rifling, the method comprising: depositing material onto a ridged outer surface of the mandrel to form the gun barrel liner, wherein the ridged outer surface corresponds to the rifling; wherein the depositing includes depositing the materials with different porosity, with inner material of relatively low porosity (less porous) and outer material of relatively high porosity (more porous).
 19. The method of claim 18, wherein the depositing includes plasma deposition; and wherein the depositing includes re-melting the inner material.
 20. The method of claim 18, wherein the inner material and the outer material have the same material composition. 